A better alternative: Use some free power management software to throttle your PC down to power energy levels when you don't need all the horsepower it has to offer. It'll save you some money and help save the earth at the same time.
Granola is a power management program which automatically adjusts your CPU's speed based on the task you're performing. If you are idling on the desktop or writing a report in your word processor, Granola eases your CPU down. But start crunching numbers in a spreadsheet or playing a game, and it'll light up your processor to accommodate.
It's available for both Windows and Linux.
The developer claims that Granola imposes essentially no overhead on your PC, so it won't affect performance -- but you stand to save quantifiable levels of power and money over the course of the year. When you install the program, it makes an initial estimate for how much it will save -- mine was about $25 -- and it adjusts that estimate over time as it regulates your computer's power consumption. Your mileage, of course, will vary. That doesn't sound like a lot, but if you have several PCs, it adds up. And it's free, so not using Granola is like throwing away money.